With 2 windows, 2 scenes, linked objects:
- enter editmode in 1 window.
- the other window allowed to enter editmode too.
- and crash happened on exit editmode.
Since editmode is in Context (scene->obedit) a bad conflict arises.
New function BKE_object_is_in_editmode() returns this info outside of
context. Note I didn't use BMEdit_FromObject() because of the assert().
NOTE: contextual storage of editmode could need rework... five places:
- ob->mode / ob->restore_mode
- scene->object
- CTX_data_edit_object()
- BKE_object_is_in_editmode()
- view3d mode handling menu
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